To the student these essays have no other interest than is due to the fact that they are Davy’s first contribution to the literature of science. No beginning could be more inauspicious. It is the first step that costs, and Davy’s first step had well nigh cost him all that he lived for. As additions to knowledge they are worthless; indeed, a stern critic might with justice characterise them in much stronger language. Nowadays such writings would hopelessly damn the reputation of any young aspirant for scientific fame, for it is indeed difficult to believe, as we read paragraph after paragraph, that their author had any real conception of science, or that he was capable of understanding the need or appreciating the value of scientific evidence.
The essays are partly experimental, partly speculative, and the author apparently would have us believe that the speculations are entirely subservient to and dependent on the experiments. Precisely the opposite is the case. Davy’s work had its origin in Lavoisier’s “Traité Elémentaire,” almost the only text-book of chemistry he possessed. Lavoisier taught, in conformity with the doctrine of his time, that heat was a material substance, and that oxygen was essentially a compound body, composed of a simple substance associated with the matter of heat, or caloric. The young novitiate puts on his metaphysical shield and buckler; and with the same jaunty self-confidence that he assailed Locke and criticised Berkeley, enters the lists against this doctrine, determined, as he told Gregory Watt, “to demolish the French theory in half an hour.”
After a few high-sounding but somewhat disconnected introductory sentences, and a complimentary allusion to “the theories of a celebrated medical philosopher, Dr. Beddoes,” he proceeds to put Lavoisier’s question, “La lumière, est-elle une modification du calorique, ou bien le calorique est-il une modification de la lumière?” to the test of experiment. This he does by repeating Hawksbee’s old experiment of snapping a gunlock “armed with an excellent flint” in an exhausted receiver. The experiment fails in his hands; such phenomena as he observes he misinterprets, and he at once concludes that light and heat have nothing essentially in common. “Nor can light be as some philosophers suppose, a vibration of the imaginary fluid ether. For even granting the existence of this fluid it must be present in the exhausted receiver as well as in atmospheric air; and if light is a vibration of this fluid, generated by collision between flint and steel in atmospheric air, it should likewise be produced in the exhausted receiver, where a greater quantity of ether is present, which is not the case.” Since, then, it is neither an effect of caloric nor of an ethereal fluid, and “as the impulse of a material body on the organ of vision is essential to the generation of a sensation, light is consequently matter of a peculiar kind, capable when moving through space with the greatest velocity, of becoming the source of a numerous class of our sensations.”
By experiments, faultless in principle but wholly imperfect in execution, he next seeks to show that caloric, or the matter of heat, has no existence. His reasoning is clear, and his conceptions have the merit of ingenuity, but any real acquaintance with the conditions under which the experiments were made would have convinced him that the results were untrustworthy and equivocal; and yet, in spite of the dubious character of his observations, he arrived at a theory of the essential nature of heat which is in accord with our present convictions, and which he states in the following terms:—
“Heat, or that power which prevents the actual contact of the corpuscles of bodies, and which is the cause of our peculiar sensations of heat and cold, may be defined a peculiar motion, probably a vibration, of the corpuscles of bodies, tending to separate them.”
This conception of the nature of heat did not, of course, originate with him, and it was a question with his contemporaries how far he was influenced by Rumford’s work and teaching. On this point Dr. Beddoes’s testimony is direct and emphatic. He says:—
“The author [Davy] derived no assistance whatever from the Count’s ingenious labours. My first knowledge of him arose from a letter written in April 1798, containing an account of his researches on heat and light; and his first knowledge of Count Rumford’s paper was conveyed by my answer. The two Essays contain proofs enough of an original mind to make it credible that the simple and decisive experiments on heat were independently conceived. Nor is it necessary, in excuse or in praise of his system, to add, that, at the time it was formed, the author was under twenty years of age, pupil to a surgeon-apothecary, in the most remote town of Cornwall, with little access to philosophical books, and none at all to philosophical men.”
Having thus, with Beddoes, expunged caloric from his chemical system, Davy proceeds to elevate the matter of light into its place. According to Lavoisier oxygen gas was a compound of a simple substance and caloric; Davy seeks to show that it is a compound of a simple substance and light. He objects to the use of the word “gas,” since, according to French doctrine, it is to be taken as implying not merely a state of aggregation but a combination of caloric with another substance, and suggests therefore that what was called oxygen gas should henceforth be known as phosoxygen. His “proofs” that oxygen is really a compound of a simple substance with “matter in a peculiar state of existence” are perhaps the most futile that could be imagined. Charcoal, phosphorus, sulphur, hydrogen, and zinc were caused to burn in oxygen; light was evolved, oxides were formed, and a deficiency of weight was in each case observed. He regrets, however, that he “possessed no balance sufficiently accurate to determine exactly the deficiency of weight from the light liberated in different combustive processes.”
“From these experiments, it appears that in the chemical process of the formation of many oxyds and acids, light is liberated, the phosoxygen and combustible base consumed, and a new body formed.... Since light is liberated in these processes, it is evident that it must be liberated either from the phosoxygen or from the combustible body.... If the light liberated in combustion be supposed (according to Macquer’s and Hutton’s theories) to arise from the combustible body, then phosoxygen must be considered as a simple substance; and it follows on this supposition, that whenever phosoxygen combines with combustible bodies, either directly or by attraction from any of its combinations, light must be liberated, which is not the case, as carbon, iron and many other substances, may be oxydated by the decomposition of water without the liberation of light.”
Davy is here on the horns of a dilemma, but he ignores the difficulty, and, with characteristic “flexibility of adaptation,” proceeds to offer synthetical proofs “that the presence of light is absolutely essential to the production of phosoxygen.” The character of the “proofs” is sufficiently indicated by the following extracts:—